Why mike the helium balloons float while the others rest on the table

Helium is a chemical element of the atomic number 2, an inert gas that is the lightest member of the noble gas series.

These means, Helium is lighter than air, and for this is why all other balloons may not float.

The helium balloon displaces an amount of air (Just like an empty water bottle displaces an amount of water). As long as the weight of the helium plus the balloon fabric is lighter than the air displaces, the balloon will float in the air.

water is used to store them because it would keep the radiation from getting out in the open and causing harm to the workers

the seeing the electronic configuration we can say that the element is fluorine.

fluorine is the most electronegative element in the periodic table.

but by only seeing the configuration we cannot say about its reactivity because we don't know if its configuration after excitation or by addition of electrons.

but if not then, it is the lightest halogen and exists as a highly toxic pale yellow diatomic gas at standard conditions. as the most electronegative element, it is extremely reactive: almost all other elements, including some noble gases, form compounds with fluorine.